This invention relates to flextensional microphones which are made up of a piezoelectric substrate having opposing surfaces, typically parallel surfaces when the substrate is crystalline or ceramic, and at least one sound receiving surface physically tied to the piezoelectric substrate. The microphones are at least partially isolated via a biocompatible material, e.g., by a covering or a coating. The inventive microphones may be subcutaneously implanted. The inventive microphones may be used as components of surgically implanted hearing aid systems or as components of hearing devices known as cochlear implants. Preferably the microphones are used in arrays and when used as a component of a hearing assistance or replacement device, are preferably used in conjunction with a source of feedback information, preferably another microphone. The feedback information usually relates to sound re-emitted from physical portions of the ear, e.g., the eardrum, where those portions have been directly or indirectly driven by the actuator of the implanted hearing aid.
For an implantable hearing device to transmit acousto-mechanical signals to the middle-ear or the inner ear, or electrical signals to an inner ear electrode, a microphone is needed to sense environmental sounds. To make the hearing device fully implantable, the microphone and associated wiring must be placed under the skin. Subcutaneous placement of the microphone allows the entire hearing device, i.e., that microphone, the output transducer, the battery, and associated sound processor to be implanted entirely inside the body. Fully implanted hearing devices have the important cosmetic advantage of being entirely invisible.
The inventive microphones may also be used as a component of a partially implantable hearing aid system. In a typical partially implantable hearing aid, the microphone and output transducer are implanted in the body but the power supply and sound-processing electronics are outside the body. Communication from the microphone sound processor is achieved with implanted coils using RF techniques.
Others have proposed implanting microphones into the body as a part of a hearing aid. Several microphone implantation methods have been proposed. These devices fall into two generic classes. In the first such class, the microphone is implanted subcutaneously. In the other group, the microphone is placed outside the skin and the signal is sent trans-cutaneously by a pair of coils. Our inventive microphones are generally used as subcutaneous microphones, although obviously, they have other uses.
In the first noted class of hearing aids, those using subcutaneous microphones, the transducers fall into at least four basic categories. In the first, a commercially available electret microphone is used. The electret microphone is encased and sealed in an acoustic chamber thereby making it compatible for implantation in tissue. This approach was originally described in: Kodera, K., Suzuki, K., and Ohno, T. (1988). xe2x80x9cEvaluation of the implantable microphone in the cat,xe2x80x9d in Suzuki, J.-I., editor, Middle Ear Implant: Implantable Hearing Aids, pages 117-123. Karger, Basel. More recently, such a method is found in U.S. Pat. No. 5,814,095, to Willer et al. and in U.S. Pat. No. 5,859,916, to Ball et al.
In another method, the vibrations of the malleus are sensed by a piezo transducer. This approach is suggested in U.S. Pat. No. 5,531,787, to Lesinski et al.; U.S. Pat. No. 5,788,711, to Lehner et al.; U.S. Pat. No. 5,842,967, to Kroll; and U.S. Pat. No. 5,836,863, to Bushek et al.
In yet a third method, sound vibrations in the ear canal are sensed by a PVDF (Kynar) based piezo transducer placed in the concha. This approach is shown in U.S. Pat. No. 5,772,575, to Lesinski et al.
Finally, U.S. Pat. No. 5,782,744, to Money, describes a sensor placed in the middle ear cavity to transduce the sound produced by the eardrum, or in the cochlea to transduce the fluid pressure produced by stapes motion.
In each of these techniques, the sensing microphone has been placed in various locations within the auditory periphery.
None of these documents show the use of our inventive microphone and particularly not within the array or hearing device described herein.
The inventive microphone is an acousto-active device made up of an acousto-active substrate having a pair of opposed planar surfaces. The substrate, typically made from piezoelectric single crystals (SCP) or ceramics such as PZT, PLZT, PMN, PMN-PT, have a 3 direction orthogonal to the planar surface defined by the 1 and 2 directions parallel to the planar surfaces. These materials generate a voltage measurable between the two planar surfaces when the material is strained or stressed in at least one of said three directions. The coefficients of d33, d31 and d32 commonly relate the induced voltage induced to the induced strain. In regards to the coefficient dij, the ij subscripts denote the orthogonal coordinate system. The substrate itself may be a single crystal, a single layer, or may be a multi-layer composite. Most preferred, the substrate is a single crystal. The substrate typically is generally circular although it need not be. In certain circumstances, the substrate may have at least one linear edge, e.g., it may be rectangular.
The acoustic stress is applied to the substrate by at least one stress-inducing member attached to the substrate. One of the stress-inducing members induces stress across at least one of the directions in the 1-2 planar surface having piezo coefficients d31 or d32 when a flat portion of the member is exposed to an acoustic pressure. Another stress-inducing member is also attached to the other side of the substrate, but it need not be a sound receiving member.
The microphone preferably is isolatable from the surrounding body using a biocompatible material, perhaps a covering, casing, or bag over at least a portion of the stress-inducing members. It is highly preferable that the substrate be capable of producing a detectable voltage across its planar surfaces when the first stress-inducing member is subjected to a sound in the audible frequency range (100 Hz-100 kHz), and levels of 40-120 dB corresponding to a microphone sensitivity of 0.2 mV/Pa to 50 mV/Pa and a noise figure of less than 40 dB SPL (Sound Pressure Level).
The system including the inventive transducer may further include a voltage receiver, e.g., a detector, an A/D converter, an amplifier, or the like, for receiving the voltage generated across the substrate surfaces when the stress-inducing members are exposed to sound or to vibrations due to sound. The voltage produced as a result of the stress applied to the substrate is measured across electrodes placed on the substrate surfaces. The electrodes may be independent, may be an adhesive affixing the stress-inducing members to the substrate, or may be the stress-inducing members themselves. The electrodes may be metallic or a conductive polymer.
The first or primary stress-inducing member generally includes a sound receiving diaphragm generally parallel to the adjacent substrate planar surface. The sound forces impinging on the sound receiving diaphragm are transmitted to the substrate via any of a number of structures. The preferred structure is a frusto-conical shell section (a xe2x80x9ccymbalxe2x80x9d) further having an outer lip fixedly attached to the substrate. Other structures include frusto-hemispherical shell sections (a xe2x80x9cmooniexe2x80x9d), bridge shaped components having at least two linear spacing members attached both to the sound receiving diaphragm and to the substrate, and prismatoid shell sections. Other structures are also suitable.
The inventive device may be included in an array of microphones or used as a singlet. The preferred array is linear, i.e., the microphones are in a line and the sound receiving diaphragms all point in the same direction.
Furthermore, the inventive method for detecting audible sound typically comprises the steps of placing in the path of an audible sound, at least one inventive flextensional microphone that is at least partially isolated with a biocompatible coating. It is desirable that the microphone be subcutaneously implanted. It should produce a first electric signal related to the audible sound which is amplified and introduced to an output actuator coupled to a human ear component.
The flextensional microphones are preferably situated in an array to allow detection of the direction of a path of said audible sound.
It is also desirable to use an independent microphone situated so that it can hear sound re-radiated by an human ear component, e.g., the eardrum, and produce a feedback signal related to that re-radiated sound. The feedback signal is then compared to the signal sent from the microphone array and then is used to modify the amplified signal to produce a feedback-free signal for the output actuator.